Stabilizing solution-processed metal oxide thin-film transistors via trilayer organic–inorganic hybrid passivation

Utilizing printed electronics is a promising way to realize the Internet of Things (IoT) society because of its high throughput. In particular, thin-film transistors (TFTs) based on printable semiconductors are regarded as key devices toward developing trillions of sensors and radiofrequency identification tags, thereby realizing information exchange between different objects. Among different printable materials, solution-processed amorphous metal oxide semiconductors (AOSs) have been considered as potential candidates for printing TFTs owing to their high carrier mobility and excellent uniformity.¹ However, their sensitivity toward surrounding molecules and environments, such as variable mobility, as well as threshold voltage and bias stress instability, makes AOS-based TFTs unstable during fabrication processes and practical operations.^2–4 Such problems are more pronounced for TFTs based on solution-processed AOSs because of their lower active layer quality.^5–7 Specifically, different from vacuum-processed AOSs, solution-processed AOS films comprise numerous voids and organic residues, and they generally have low densities and are thin.^8–11 Hence, device stability should be investigated to print advanced electronics toward the realization of the IoT society with solution-processed AOSs.

Passivation is a simple and effective way to improve device stability by curbing the penetration of surrounding molecules, including oxygen and water, to the channel layers.¹² Some representative encapsulants, including insulating inorganics, such as SiO_x,^13,14 SiO_x/SiN_x,¹⁵ AlO_x,^16–19 and YO_x,²⁰ exhibit low gas and water permeability. However, TFT performance degradation has been observed after their deposition processes, such as plasma-enhanced chemical vapor deposition (CVD), atomic layer deposition (ALD), and sputtering.^17,21,22 Another encapsulant is insulating polymers, such as CYTOP,^21,23 parylene,²⁴ and some reactive resins,^25–27 which are compatible with mild processing conditions, and thus generate inconsiderable process-induced damage. However, such polymers exhibit insufficient protection abilities owing to their high gas and water penetration rates compared with their inorganic counterparts.²² Some hybrid systems, such as using a bilayer structure^22,28,29 and organic–inorganic hybrid polymers,^30–32 have been developed to take advantage of both materials. However, many passivation techniques used on vacuum-processed AOSs could not be directly adopted for solution-processed AOSs because of their high environmental sensitivity. Therefore, it is necessary to develop harmless and effective passivation methods suitable for solution-processed AOS TFTs.

In this study, a poly(methyl methacrylate) (PMMA)/parylene/AlO_x hybrid passivation technique was developed to simultaneously obtain strong protection without performance degradation. With this hybrid structure, a thin PMMA/parylene bilayer acted as a buffer from damage resulting from the ALD process for AlO_x, and the resultant AlO_x had a strong isolation effect. This facile and highly effective passivation technology will be suitable for facilitating the realization of printed electronics in the future.

Indium zinc oxide (IZO) was used as the active layer in this study. An IZO precursor was prepared by mixing 0.1M In(NO₃)₃ · xH₂O (Aldrich) and Zn(NO₃)₂ · 6H₂O (Aldrich) solutions (2-methoxyethanol as a solvent) at an In/Zn mole ratio of 3:2. A bottom-gate, top-contact IZO TFT with the hybrid passivation layer is illustrated in Fig. 1(a). Gate electrodes (Cr/Au/Cr, 5/25/5 nm) were fabricated by means of a conventional lift-off process on a glass substrate. A gate dielectric layer (AlO_x, 75 nm) was formed via ALD. Thereafter, the IZO precursor was spin-coated at 5000 rpm for 30 s and annealed at 370 °C for 1 h. Subsequently, photolithography was performed using a photosensitive dielectric material (PDM, Taiyo Ink Mgf. Co., Ltd.)³³ to pattern the IZO active layer, as well as source and drain (S/D) electrodes (Al, 40 nm). Furthermore, the IZO was etched with oxalic acid (0.075 mg/100 ml), and the S/D electrodes were formed via a lift-off process. The details of fabrication can be found in our previous paper.³⁴ For passivation, PMMA (Mw: 120 000) was formed using the spin-coating technique with a butyl acetate solution (5 mg/100 ml), followed by annealing at 150 °C for 1 h. Parylene (25 nm) and AlO_x (40 nm) were deposited via CVD and ALD, respectively. The ALD process was carried out with maintaining the substrate temperature at 110 °C. After both CVD and ALD, post-annealing process was not performed. All electrical measurements were conducted under ambient and dark conditions using a semiconductor parameter analyzer (Keithley 4200-SCS). The thickness of the IZO active layer and PMMA was estimated by means of x-ray reflectometry (XRR, Rigaku SmartLab with CuKα irradiation, λ = 1.54056 Å). Atomic force microscopy (AFM) was conducted using a Shimadzu SPM-9700HT.

Optical micrographs of an as-fabricated IZO TFT with a channel length (L) and width (W) of 10 µm and 200 µm, respectively, are shown in Fig. 1(b). Here, we discuss different passivation structures, including PMMA single layer, parylene single layer, AlO_x single layer, PMMA/parylene bilayer, PMMA/AlO_x bilayer, and PMMA/parylene/AlO_x trilayer. The transfer characteristics were measured before and after passivation to evaluate the process-induced performance degradation. Figure 2(a) summarizes the transfer curves of the TFTs before and after being passivated with PMMA, parylene, and AlO_x single layers. Before passivation, the TFT had a saturation mobility (μ_sat) of 2.3 cm² V⁻¹ s⁻¹, on/off current ratio (I_on/I_off) of ∼10⁸, and threshold voltage (V_th) of 2.0 V. Parylene passivation led to a large negative V_th shift, which might be caused by the long-time vacuum state during parylene deposition and/or reactive radical species derived from the parylene source that might react with weakly bonded oxygen atoms. The TFT with AlO_x passivation performed similarly, but worse than the parylene-passivated one. This may be due to the harsher processing conditions, including high vacuum together with elevated temperature (110 °C) and/or using water as a reaction species. A similar phenomenon has been reported for vacuum-processed AOSs.^17,22 Hence, ALD and parylene-coating processes tend to vary the TFT properties based on solution-processed AOSs. Nonetheless, after PMMA passivation, TFT still exhibited desirable properties, including μ_sat of 2.0 cm² V⁻¹ s⁻¹, I_on/I_off ≈ 10⁸, V_th of 1.7 V, and negligible hysteresis, thereby indicating the suitability of PMMA as a direct encapsulant for the IZO active layer. This result is attributable to the mild fabrication conditions of PMMA, i.e., the spin-coating method under ambient conditions. However, the high moisture and gas penetration rates render PMMA insufficient for passivation.^29,35,36 To solve this problem, we used PMMA as a buffer layer and deposited either parylene or AlO_x on top of it. The transfer characteristics of the resulting PMMA/parylene and PMMA/AlO_x bilayer passivation layers are summarized in Fig. 2(b); for the former, only minimal changes are observed, suggesting that the PMMA layer can effectively protect IZO TFTs during parylene deposition, despite its thickness being only 13 nm [Fig. 2(c)]. Thus, the flat and pinhole-free structure of the PMMA layer is suitable for preventing radical species from penetrating into the IZO layer [Figs. 2(d) and 2(e)]. On the one hand, the PMMA buffer layer was unable to provide sufficient protection during the ALD of AlO_x because of the harsher deposition conditions of ALD compared to that of the parylene coating. On the other hand, the PMMA/parylene bilayer structure acted as a strong buffer layer for the TFTs during the ALD of AlO_x, whereby evident TFT performance degradation was not observed [Fig. 2(f)].

To further understand the isolation ability of PMMA and PMMA/parylene buffer layers, their waterproof properties were evaluated by immersing the TFTs in deionized water for 15 min, air-dried, and then measured and compared the transfer characteristics of different devices [Fig. 3(a)]. The transfer curves of TFTs with PMMA, PMMA/parylene passivation, and TFT without passivation after water treatment are shown in Fig. 3(b). A TFT without passivation and water treatment was used as a reference. Without passivation, a considerably negative V_th shift and large hysteresis were observed because of the large amount of water existing on the backchannel layer acting as the donor. The large hysteresis might also be generated because of the high-humidity conditions that could cause additional effects, such as peeling of the active layer, thereby resulting in a severe deterioration in TFT performances.⁵ Conversely, TFTs with passivation layers exhibited a smaller V_th shift and negligible hysteresis. Compared with the PMMA single layers, PMMA/parylene bilayers exhibited stronger protection abilities. This result is consistent with the phenomenon that the PMMA/parylene bilayer has better buffer functions during AlO_x deposition than the PMMA single layer because of the low moisture penetration rate of parylene.²⁴ 

In this study, a hybrid trilayer passivation method was developed to realize the high stability of TFTs based on solution-processed IZO. With the help of a thin PMMA buffer layer, process-induced damage during parylene deposition was perfectly precluded. In addition, owing to the strong protection of the PMMA/parylene buffer layer, performance deterioration during AlO_x deposition via ALD was effectively suppressed. Owing to the high isolation effect of the hybrid passivation layer, the IZO-based TFTs were significantly stabilized under bias stress in air, whereby the I_D drop rate was reduced from 67% in the non-passivated TFT to 2%, while ΔV_th decreased from 2.8 V to 0.2 V. Therefore, the present passivation technique provides a near-ideal protection of solution-processed AOS TFTs, and it could improve the future availability of printed electronics for the realization of the IoT society.

The data that support the findings of this study are available from the corresponding author upon reasonable request.